[Style]: move string extension method to StringExtension.cs

[Wi1l-B0t]

Description

Add string extension methods: GetStrictUTF8String GetStrictUTF8Bytes GetStrictUTF8BytesCount.
Move some string extensions methods to StringExtension.cs.

Fixes # (issue)

Type of change

• Optimization (the change is only an optimization)
• Style (the change is only a code style for better maintenance or standard purpose)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[cschuchardt88]

Since this is extensions get doesn't make sense here. AsStrictUtf8String is a better name. Make sure UTF isnt capital letters. It should be Utf

[Wi1l-B0t]

OK

[Wi1l-B0t]

Done